1. Field of the Invention
This invention relates to vertical cavity semiconductor optical amplifier (VCSOA) technology for free space optical communication, in particular to arrays of active modulating retro-reflectors.
2. Brief Description of Prior Art
In a co-pending U.S. patent application Ser. No. (10/621,208), it has been disclosed that photoreceivers using vertical cavity semiconductor optical amplifiers (VCSOA) as optical preamplifiers is capable of quantum limit sensitivities for free space optical communications networks. It has also been disclosed that all the components of a photoreceiver-optical amplifier, optical band-pass filter, and photodiode module can be monolithically integrated on a single chip. In this invention, we apply the optical preamplified receiver technology to modulating retro-reflectors (MRR), either as active MRR or as optically preamplified MRRs with which higher speeds, higher extinction ratios and longer ranges can be enabled for the same transmit power.
A retroreflector, often referred to as a corner cube, is an optical device that redirects an incoming light back along the direction of incidence. When a mechanism is added at the aperture of the retroreflector to modulate the incoming signal, the device is called an MRR. Although various modulating retro-reflectors such as the multiple quantum well (MQW) retroflectors, micro-electro-mechanical corner cube retroreflector, ferroelectric liquid crystal modulator, thin film diffractive modulating retroreflector, have been demonstrated (1,2,3), their performance is still limited to lower bandwidths and short haul ranges. One of the main drawbacks of current MRRs is the relatively large capacitance resulting from the need for large area and high retroreflected power. In addition, the high series resistance needed for large modulation depths limits the bandwidth Other drawbacks are: high cost, high losses, slow speed, limited bandwidth, low reflection efficiency.
For example Gilbreath, G. C., Bowman, S. R., Rabinovich, W. S., Merk, C. H., and Senasack, H. E have proposed a Modulating Retroreflector Using Multiple Quantum Well Technology (U.S. Pat. No. 6,154,299). The MQW modulators are based on PIN diodes, in which the intrinsic region consist of several (up to one hundred) very thin (−10 nm) alternate layers of narrow bandgap semiconductor material as the well and wide bandgap semiconductor material as the barrier. Since the wells are thin, the bands are quantized and the exciton induces a sharp absorption feature at a wavelength that is determined by a constituent materials and the structure. When a reverse bias is applied to the device, the electric field changes the exciton absorption feature shifting it to longer wavelengths with a drop in magnitude. Therefore, by varying the voltage to the device, the transmission near the band edge changes dramatically and the device can serve as an on-off shutter. The modulator is designed to work in transmissive or reflective mode at a wavelength of 1550 nm, compatible with many laser diode sources. However, the MRRs based on MQWs are passive and have as much a 3 dB reflective loss. The proposed VCSOA technology enables amplified retroreflected power, thus alleviating the range/bandwidth/modulation trade-offs of MRRs. In this invention, we have devised an approach for implementing a compact active MRR that involves either using the VCSOA or monolithic integration of VCSOA with optical modulator.